Spark plug



United States Patent 3,133,223 SPARK PLUG Marion Mallory, Detroit, Mich., assignor to The Mallory Research Company, Detroit, Mich., a corporation of This invention relates to spark plugs for use in internal combustion engines, and more particularly to spark plugs of the craw type in which the area of the insulator exposed to the mixture of gases in the cylinder is reduced to a minimum.

Spark plugs of the crawl type have heretofore been proposed. In such plugs, the spark, instead of jumping an air gap from the electrode to ground, flashes over the surface of the insulator.

One problem encountered with such plugs has been that the insulator surface has tended to become coated with moisture and other materials contained in the combustible mixture of the engine cylinder. This coating has provided a low resistance path for current and has resulted in a leakage of current from the electrode to ground. Such leakage occurs at low voltage and does not provide the desired high-temperature spark to cause combustion of the combustible mixture at the proper time. In such plugs, a relatively large area of the inner end of the insulator is exposed to the fuel mixture and the electrode extends from the inner end of the insulator to a point a short distance therebeyond.

It is, therefore, an object of my invention to provide a spark plug of the crawl type in which only a very small portion of the insulator surface is exposed to the fuel mixture, thus preventing current leakage and assuring the proper electrical flash to cause ignition of the fuel mixture.

Other objects of this invention will appear in the follow-- ing description and appended claim, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.

FIGURE 1 is a side elevational view in section of my improved spark plug.

FIGURE 2 is an enlarged cross sectional View of the lower portion of my improved spark plug.

Before explaining the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

As will be noted in the figure, my spark plug comprises an annular metallic shell 12, the inner end 14 being externally threaded for securement of the plug on an engine cylinder head. When so secured, the shell forms a ground. The shell 12 is provided with a bore therethrough comprising a first enlarged section 16 having a shoulder 18, a second reduced section 20 which is spherically contoured at 22 to receive the inner end of an elec trical insulator body 24 and a third reduced section 26 which extends from bore section 20 to the inner end of the shell. A wrench-engageable head 28 is provided on the shell for threading the shell into the engine cylinder head.

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The electrical insulator body 24, which is preferably fabricated from porcelain, is received in the shell. An annular shoulder 30 is provided on the insulator to seat on the shoulder 18 of the shell. The inner end of the insulator is contoured to mate with the bore section 20. A thin film of a sealing compound is provided on the inner end 32 of the insulator to block the flow of current over the surface thereof. The upper end 34 of the insulator extends outwardly from the shell.

The insulator body has a bore therethrough comprising a first enlarged section 40, a second reduced section 42 and a third reduced section 44. The bore section 40 is internally threaded at 46 to threadingly receive electrical terminal 48. An electrode 50 is secured within the insulator bore. The upper end 52 of the electrode is separated from the lower end 54 of the terminal to provide an air gap 55 therebetween. This gap acts as a spark gap to intensify the voltage applied to the electrode during operation of the spark plug. An annular enlargement 56 is provided intermediate the ends of the electrode and seats against a shoulder 58 formed by the juncture of bore sections 42, 44. The inner end 60 of the electrode terminates at a point short of the inner end of the insulator bore section 44 to provide a recess 62 in the bore separating the shell 12 from the electrode.

The insulator 24 is secured within the shell 12 by cement 64. The upper end of the shell is turned down at 66 to secure the cement in place. The electrode 50 is secured in place by cement 68 to thus secure the entire assembly.

In operation, a voltage is applied to the terminal 48, jumps the gap 55 to the electrode and causes a relatively high voltage to be present at the inner end 60 of the electrode. This voltage eventually overcomes the resistance of the insulator walls 70 to cause a spark to flash from the electrode to the shell with resultant ignition of the cylinder fuel mixture.

Because of the small surface area 70 of the insulator exposed to the fuel mixture, a surface film does not form on the insulator to reduce the resistance of the insulator gap. Current cannot leak back up over the insulator because of the sealing compound around the lower end of the insulator. It may be noted at this point that instead of a sealing compound, a gasket may be provided around the lower end of the insulator to serve as a block to current leakage over the insulator surface.

It will be noted that the inner end of the insulator terminates at a point short of the inner end of the shell 12. In this way, the shell 12 covers the entire inner end of the insulator excepting for the bore opening. The bore opening can be, if desired, made smaller than the diameter of the electrode to further increase the resistance of the insulator gap.

Having thus described my invention, I claim:

In a spark plug comprising an annular metallic shell having an inner end with external threads for the securement of the plug on an engine cylinder head; said shell having a bore therethrough; an electrical insulator body secured within the bore of the shell; the inner end of the insulator body terminating at a point short of the inner end of the shell; said insulator body having a bore therethrough; an electrical terminal secured in the bore of the insulator body at the outer end thereof; the improvement comprising the provision of electrically-insulating sealing material on the inner end of the insulator; said sealing material extending over the insulator body surface from the bore opening; an electrode in the insulator bore terminating at a point short of the inner end of said bore to provide a recess in said bore separating the shell from the electrode; the insulator surface defining said recess forming the shortest electrical path between the shell and the electrode whereby normal spark plug flash will travel across said surface.

References Cited in the file of this patent UNITED STATES PATENTS Besson June 16, Klever May 25, Schwetger Feb. 15, Stephenson June 20, Hall Feb. 10, 

